Oops, Have I been There??
I believe I may have had 2 such encounters in 73NGs whilst transitting through what we as a crew believed were innocuous remains of old cunimbs with little or no radar echoes at our level.
Symptoms: Light chop, reduced vis, SAT-55c but didn't note the TAT beyond not needing engine anti-ice, speed began to reduce, aft-facing side of winscreen wiper assembly, normally matt black, turned white indicating some form of accretion NOT airflow impacted.
Quiet panic set in, don't recall anything like that in the Met theory, engines to MCT, switch on engine anti-ice for fear of what may happen to them if not protected. Avoided wing de-icing due double bleed-trip caution in NNC above FL350, speed got to touch amber bar, but luckily fell out the far side of the line of diffuse tops without stalling practice and further abnormalities.
My theory about the "lee side" ice accretion is that our airframe is probably charged up with static electricity by air friction and we've provided an attractive parasitic host to several billions of ice crystals oppositely charged after being spat out by a previously active, now decayed cumulo-nimbus.
Following the 2nd episode I submitted ASR and asked for the FDR to be pulled for investigation but never got any feedback and the company folded not long after. (Not my fault gov!)
Hope this helps a little?